CellRendererInvoker._redraw_path(): assure column is defined

We know that our code is correct and column should always be assigned, but it
never hurts to act defensively and guard against bugs in other pieces of the
code (or other components like GTK).

Reviewed-by: James Cameron <quozl@laptop.org>
Acked-by: Simon Schampijer <simon@laptop.org>
CC: Aleksey Lim <alsroot@member.fsf.org>

src/sugar/graphics/palettewindow.py

@@ -938,9 +938,11 @@ class CellRendererInvoker(Invoker):
             self.notify_mouse_leave()
 
     def _redraw_path(self, path):
+        column = None
         for column in self._tree_view.get_columns():
             if self._cell_renderer in column.get_cell_renderers():
                 break
+        assert column is not None
         area = self._tree_view.get_background_area(path, column)
         x, y = \
             self._tree_view.convert_bin_window_to_widget_coords(area.x, area.y)